PCBA processing paste selection of the "five-dimensional evaluation method"

In PCBA processing, solder paste selection is a key link in determining solder quality, reliability and productivity.The "5-Dimensional Evaluation Method" is a systematic and structured selection method, which comprehensively considers the five core dimensions of alloy composition and performance, process adaptability, solder reliability, compatibility and applicability, and cost-effectiveness, to help select the most suitable paste for the specific product and process requirements.The following is a detailed description of these five dimensions and suggestions for optimisation：

I. Alloy composition and performance

Alloy composition is the basis of solder paste performance, which determines the melting point, wettability, mechanical strength, thermal fatigue resistance, electrical and thermal conductivity, and cost.

Lead-free and leaded options: According to the requirements of environmental regulations (e.g. RoHS, REACH), lead-free solder paste has become the mainstream.Among the lead-free solder pastes, SAC alloys (e.g. SAC305, SAC387) are widely used due to their excellent overall performance.Low-silver alloys (e.g. SAC0307) have lower costs, but are slightly less wettable and reliable, and are suitable for scenarios that do not require high reliability.Bismuth-containing alloys have a low melting point and are suitable for heat-sensitive components, but have poor ductility and need to be chosen carefully.

Specific alloy types:

SAC305: melting point 217-219°C, good wettability, high mechanical strength, is the most versatile lead-free solder paste.

Low silver alloy: such as SAC0307, lower cost, but high temperature reliability may be insufficient.

Bismuth-containing alloys: e.g. Sn42Bi58, with a melting point of 138°C, it is suitable for heat-sensitive components such as LEDs, but its drop resistance is weak.

High reliability alloys: such as SAC387, add trace elements (such as Ni, Ge), thermal fatigue, drop resistance significantly improved, suitable for automotive, aerospace and other fields.

Melting point range: need to match with component heat resistance, PCB Tg value, reflow temperature profile to avoid thermal damage.

Wettability: affects the quality and appearance of solder joint formation, the higher the silver content, the better the wettability, but the higher the cost.

Mechanical strength and reliability: high reliability applications need to focus on tensile strength, shear strength, thermal cycling / drop impact resistance.

II. process adaptability

Process adaptability means that the physical properties of the solder paste (e.g. viscosity, thixotropy, collapse) must be perfectly matched with the production process (printing, placement, reflow).

Printability:

Viscosity and thixotropy: The right viscosity and good thixotropy are the key to perfect release of the stencil and the formation of a clear and full solder paste pattern.Fine pitch components require more demanding printability.

Resistance to collapse: After printing and before reflow, the paste pattern should remain clear without excessive spreading or bridging.

SMD Adhesion: The solder paste needs to provide enough adhesion to hold the component securely after placement and before reflow to prevent shifting.

Reflow characteristics:

Flux Activity and Activation Temperature: Fluxes need to be activated at the right temperature to effectively remove oxides and to complete the main reaction before peak temperature.

Thermal Collapse Behaviour: Fluidity control of the paste during the warm-up and ramp-up phases to prevent premature bridging or component displacement.

Wetting speed and spreadability: affects solder joint formation and appearance.

Residue Characteristics:

Residue types: no-clean, water-clean, solvent-clean.No-clean is the mainstream, but need to ensure that the residue does not affect the appearance of the inspection and subsequent reliability testing.

Residue ionic cleanliness: conductivity or ionic contamination tests need to meet the standards to prevent leakage and corrosion.

III. welding reliability

Welding reliability refers to the ability of the welded joint to maintain electrical connection and mechanical strength under various working environment stresses (thermal cycling, mechanical vibration, shock, humidity, etc.) during the expected life of the product.

Reliability of the alloy itself: such as SAC305 reliability is better, low-silver alloys are slightly weaker, bismuth-containing alloys in the drop resistance may be worse but may be better under thermal cycling.High reliability applications require specialised alloys.

Solder joint microstructure: A good IMC layer (intermetallic compound) at the solder interface is the basis of reliability, and is affected by the alloy, temperature profile, and condition of the surface to be soldered.

Influence of flux residues: Are the residues corrosive?Are they hygroscopic?Does it cause electrochemical migration in high humidity or biased environments?

Thermal Fatigue Resistance: The ability of the solder joint to resist stress due to CTE mismatch when the product undergoes temperature changes.

Mechanical shock/drop resistance: Portable devices (e.g. mobile phones, tablets) need to be focused on.

Long-term aging performance: Overgrowth of IMC may affect reliability.

IV. compatibility and applicability

Compatibility and applicability refers to whether the solder paste is compatible with all the materials involved (components, PCBs) and the production environment.

Component terminal/pad plating: Does the solder paste have good solderability and compatibility with the plating of the component (e.g. Sn, SnBi, AgPd, Au, NiPdAu, OSP, Immersion Sn/Ag)?

Component heat resistance: solder paste melting point/reflow temperature must be lower than the maximum temperature that the component can withstand.

Special components: e.g. Aluminium electrolytic capacitors, beads, connectors, etc. may be more sensitive to temperature or flux.

PCB compatibility:

Pad Finish: Solder paste needs to be compatible with PCB pad finishes (HASL, ENIG, OSP, Immersion Silver/Tin).

PCB material and Tg value: The reflow temperature needs to be lower than the Tg value of PCB to prevent deformation and delamination.

Storage and use of the environment: solder paste storage conditions (usually refrigerated), reflow temperature requirements, the use of environmental temperature and humidity to meet?

Equipment compatibility: Is it applicable to existing printing machine (squeegee type, speed, pressure), mounter, reflow oven (does it need nitrogen?)??

Regulatory and standards compliance: Does it meet the environmental regulations of the product's target market?Does it comply with industry or customer specific standards?

V. Cost-effectiveness

Cost-effectiveness refers to the pursuit of optimal comprehensive cost under the premise of meeting the requirements of the first four dimensions.

Tin paste unit price: alloy cost is the main factor (silver price fluctuations have a big impact).Low-silver alloys and tin-bismuth alloys usually cost less than SAC305.

Process cost:

Yield impact: improper selection leads to soldering defects (bridging, false soldering, tin beads, standing monuments, etc.) will significantly increase the cost of rework and scrap.

Productivity: Solder pastes with wide printing window, good anti-collapse and high process tolerance can reduce the time for machine adjustment, stencil cleaning, etc. and improve UPH.

Nitrogen use: Some solder pastes are better soldered in a nitrogen environment, but nitrogen has a cost.

Cleaning costs: If you use solder paste that needs to be cleaned, the cost of cleaning equipment, solvents, labour and environmental treatment needs to be factored in.

Stencil Life: The cleanliness of the solder paste affects the residue on the bottom of the stencil, which may require more frequent wiping or cleaning.

Maintenance and rework costs: Is the residue easy to rework and repair?

Inventory and waste: Solder pastes have a shelf life and inventory needs to be managed to minimise expiry and waste.The service life of the tin after opening also needs to be considered.

How to apply the "5-Dimensional Evaluation Method"?

Clarify product requirements: Understand the product's application area, reliability requirements, working environment, life expectancy, and cost objectives.

Sorting out process constraints: Be clear about existing production equipment, range of process parameters, use of nitrogen, cleaning capabilities, etc.

Identify key challenges: Are there any difficulties in product design?Such as fine pitch BGA, QFN bottom heat sink pads, mixed components, heat sensitive components, shaped connectors.

Filter candidates by dimension: Based on the first three points, set basic requirements and priorities in each dimension.Initially screen several candidate solder paste brands and models that meet the basic requirements.

In-depth testing and validation:

Process window testing: Test the soldering effect under different printing parameters and reflow profiles to assess process tolerance.

Soldering quality assessment: Checking the appearance of solder joints, X-Ray inspection, and slice analysis.

Reliability Test: Conduct appropriate reliability tests according to product requirements.

Residue assessment: Inspect the appearance, conduct ionic contamination test, surface insulation resistance test.

Cost-benefit analysis: Calculate the comprehensive cost of each candidate solution by combining the test results with material and process costs.

Final Decision and Curing: Select the optimal integrated solder paste and write its specifications and process parameters into the official production process document.

Through the systematic application of the "5-Dimensional Evaluation Method", the risk of solder paste selection can be minimised, the production process can be optimised, and the solder quality and long-term reliability of PCBA products can be ensured, while achieving the best cost-effectiveness.